The rules from FIBA says:
43.2.3. The free-throw shooter shall:
• Take a position behind the free-throw line and inside the semi-circle.
• Use any method to shoot a free throw in such a way that the ball enters the basket from above or the ball touches the ring.
• Release the ball within 5 seconds after it is placed at his disposal by the official.
• Not touch the free-throw line or enter the restricted area until the ball has
entered the basket or has touched the ring.
• Not fake a free throw
I don't see any of this leaving room for a "practice" throw, as you can't even fake a free throw. And you have only 5 seconds, so there is not really time for a practice throw any way.